An image detector is generally subject to vibrations which might distort a detected image of a scene. The vibrations can be linear—where the image detector undergoes a linear displacement, as well as angular—where the image detector rotates about one or more axes. In case of an observation post, these vibrations may be caused by seismic waves, wind, or a rail car passing by. In case of an image detector mounted on a ground vehicle, the image can be distorted as a result of the vibrations of the engine and the road itself. In case of an image detector mounted on a marine vessel, the image can be distorted as a result of ocean waves. Likewise, image distortion can occur in images detected by an image detector mounted to a ground vehicle, an airborne platform, such as an aircraft, a helicopter or a satellite.
Methods for compensating for the vibrations and noise in order to obtain a stabilized image are known in the art. For example, a gyroscope connected to the image detector detects the inertial rotations of the image detector, and a servo system (including a servo motor and a controller) rotates the gimbals on which the image detector is mounted, in the opposite direction and by the same amount, according to the output of the gyroscope. The image can be further refined by employing additional gyroscopes and by providing each gyroscope additional degrees of freedom.
An alternative method for stabilizing the detected image is by processing the detected image via an image processor (i.e., correlator). The correlator detects movement of landmarks within the image, and between consecutive images, by processing the image. The correlator, then stabilizes the image by shifting the pixels in the image by the detected amount of movement, and in the opposite direction. However, the operation of the correlator is ineffective in case the signal to noise ratio (SNR) of the light reaching the image detector is low (e.g., in case of a night vision system). Furthermore, the operation of the correlator is limited to a stationary platform (e.g., an observation mast), a vehicle which moves directly toward the scene while the line of sight of the image detector remains constant, or in case a user of the moving vehicle moves the image detector to maintain a constant line of sight, despite the movements of the vehicle.
The optical assembly which is located in front of the image detector, generally includes one or more mirrors. A further alternative method for stabilizing the detected image, is by allowing all the mirrors to move as a result of the vibrations except one, and moving that mirror by an amount proportional to the vibrations and in the opposite direction.
Reference is now made to FIG. 1, which is a schematic illustration of a system, generally referenced 50, for providing a stabilized image of a scene detected by an image detector subjected to external disturbances, as known in the art. System 50 includes an image detection frame 52, a correlator 54 and a display 56. Image detection frame 52 includes an image detector 58 and gimbals 60. Gimbals 60 include a gyroscope 62, a movement processor 64 and a servo motor 66.
Image detection frame 52 is a pedestal which is mounted to an aircraft (not shown). Gimbals 60 are mechanically connected to image detection frame 52. Image detector 58, gyroscope 62 and servo motor 66 are mechanically connected to gimbals 60. Correlator 54 is electrically connected to image detector 58 and to display 56. Movement processor 64 is electrically connected with gyroscope 62 and with servo motor 66. Gimbals 60 is a three degrees of freedom (DOF) mechanism.
Gyroscope 62 is in form of a spinning solid body (not shown) having such a moment of inertia that a rotating shaft (not shown) which supports the solid body, points to a predetermined direction relative to the Earth, despite changes in direction of flight of the aircraft (i.e., an gyroscope 62 is an electromechanical gyroscope). Gimbals 60 can move about the X, Y and Z axes of a three-dimensional Cartesian coordinate system 70. Since image detector 58 is firmly connected with gimbals 60, image detector 58 can also move about the X, Y and Z axes.
Image detector 58 detects an image (not shown) of a scene 68 while being subject to random disturbances caused by the powerplant of the aircraft and aerodynamic forces. Since image detector 58 is firmly connected to the aircraft, these disturbances are transmitted to image detector 58, and thus display 56 produces a distorted image of scene 68.
Gyroscope 62 detects rotations of image detector 58 about the X, Y and Z axes (i.e., pitch, yaw and roll, respectively) due to the disturbances and produces an output angle to movement processor 64. Movement processor 64 determines a counteractive movement to be applied to image detector 58, in order to cancel out the rotations of image detector 58 due to the disturbances and to stabilize the detected image. Movement processor 64 directs servo motor 66 to move gimbals 60 about the X, Y and Z axes by the respective amplitude of disturbance and in the opposite direction, thereby mechanically stabilizing image detector 58. Display 56 displays a stabilized image of scene 68.
Alternatively or additionally, correlator 54 performs frame-to-frame inspection of the image detected by image detector 58, by employing an image processing procedure. When correlator 54 detects that the location of a landmark (not shown) in the current image of scene 68 is different than the one in the previous image, relative to the background (not shown), correlator 54 shifts the current image in a direction opposite to the detected direction and by an equal amount, along the X and Y axes. Thus, correlator 54 corrects for fine disturbances of substantially small amplitudes and substantially large frequencies (e.g., 5 microradians and 200 Hz) transmitted to image detector 58 about the X, Y and Z axes.
The performance level of correlator 54 is a function of the SNR of the video signal received by correlator 54. Therefore, correlator 54 can produce a stabilized image in the daytime when the light intensity and the SNR is large. However, this is not the case if system 50 operates in a dark environment where a substantially small amount of light photons reach image detector 58 per unit of time, and if the SNR of the signal at the input of correlator 54 is much lower than in the case of a daytime operation.
Correlator 54 can correct image disturbances caused by angular disturbances of up to a few milliradians. The degree of image stabilization of the image displayed by display 56 also depends on the frame rate of display 56. For example, at 30 frames per second, correlator 54 can correct image disturbances caused by vibrations up to three Hertz, or less than one Hertz. This is due to the limitations of the algorithm of correlator 54 at this frame rate and frequencies. Furthermore, the algorithm is not responsive to images which are substantially blurred.
In order for display 56 to produce high resolution images of scene 68 located at a large range, image detector 58 has to be stabilized to five microradians root mean square (rms) or less. However, with the arrangement of system 50 as described herein above, image detector 58 can be stabilized to between 10 and 30 microradians rms. Furthermore, acceptable images at a sufficient contrast level and detailed data, can be obtained at disturbances at frequencies of up to a few Hertz or less than one Hertz. and limited to a good images quality with enough details and contrast to observe and for external vibration frequencies of less than 1-3 Hz
Alternatively, gimbals 60 can have one DOF, in which case image detector 58 is free to rotate only about the X axis. Further alternatively, gimbals 60 can have two DOFs, in which case image detector 58 is free to rotate about the X and Y axes. For example, if the image is stabilized only against yaw of the aircraft, then gimbals 60 has one DOF. If the image is stabilized both against yaw and pitch of the aircraft, then gimbals 60 has two DOFs. If the image is stabilized against yaw, pitch and roll of the aircraft, then gimbals 60 has three DOFs.
Gimbals 60 is stabilized by employing a plurality of springs (not shown), dampener, servo motor control system (not shown), inertial sensor (not shown), encoder, resolver, potentiometer, or tachometer, in order to compensate for large amplitude disturbances. Due to limited mechanical stiffness, control system noise, and control system gain, and due to friction, backlash, and mechanical resonance, gimbals 60 have a limited stabilization bandwidth. Generally, they gimbals 60 can be stabilized against disturbances caused by wind and ocean waves (i.e., one Hz or less), by movement of a ground vehicle, flight disturbances, seismic waves between three to four Hertz, and the like. Thus, gimbals 60 can be stabilized against harmonic disturbances at an amplitude of several degrees and at frequencies less than 25 Hz. Furthermore, gimbals 60 dampen harmonic disturbances at amplitudes of less than several tens and hundreds microradians, excluding other disturbances—at higher frequencies. Vibrations caused by, for example, helicopter rotors, aircraft propellers, jet engines, a motor boat, ship, machine gun, power train of a ground vehicle, construction machinery, turning machines, milling machines, and the like, are at several tens of Hertz. In case the vibrations are greater than 50 Hz, inertial servo control assemblies are not capable to stabilize gimbals 60 at an accuracy of a few microradians.
Disturbances which are transmitted to image detector 58 after stabilizing gimbals 60 (i.e., residual disturbances) generally cause image distortion. System 50 can stabilize the detected image to high accuracies of 20-100 microradians or lower accuracies of 1000-3000 microradians, at angular disturbances of less than several tens of degrees per second, and at frequencies between zero to several Hz. However, in case accuracies of better than 5 microradians are required, at residual disturbances of 20-200 Hz and an amplitude of a few hundred microradians, system 50 is not capable to stabilize the detected image.
U.S. Pat. No. 5,754,226 issued to Yamada et al., and entitled “Imaging Apparatus for Obtaining a High Resolution Image” is directed to an imaging apparatus which provides an image of a subject at a resolution higher than that obtained by an imaging plate. An actuator changes the angle of inclination of a transparent refracting plate according to a control signal received from a control section, thereby refracting an incident image light and shifting the position of an image formed on an imaging plate. The actuator changes the angle of inclination of the transparent refracting plate, so that an image K10 horizontally shifted by half a pixel, an image K11 vertically shifted by half a pixel, and an image K11 horizontally and vertically shifted by half a pixel, with respect to a reference image K00, are formed on the imaging plate.
A synthesis section synthesizes respective images I10, I01, I11, and I00 into a single image in which the horizontal and vertical sampling frequencies are respectively doubled. A motion vector detecting section detects motion vectors of the images I10, I01, and I11, with respect to the reference image I00. The synthesis section synthesizes the four images I10, I01, I11, and I00 based on the motion vectors detected by the motion vector detecting section.
U.S. Pat. No. 6,429,895 B1 issued to Onuki and entitled “Image Sensing Apparatus and Method Capable of Merging Function for Obtaining High-Precision Image by Synthesizing Images and Image Stabilization Function” is directed to an image sensing apparatus which improves the resolution of an image of an object and stabilizes the image in a vibrating environment. A focusing actuator moves a first lens group back and forth along the optical axis to perform focus control. A focus detects the position of the first lens group. Vibration-type gyroscopes sense the angular vibration in the vertical and the horizontal direction of the image sensing apparatus.
U.S. Pat. No. 5,125,595 issued to Helton and entitled “Digital Image Stabilization System for Strapdown Missile Guidance”, is directed to a strapdown missile guidance system which maintains an image of a target within plus or minus 50% of a field of view of a sensor, at all times during the flight of a missile. A rate gyro measures the rate of motion of the missile and produces a rate output. A pick-off device constantly measures the angle between the missile and the sensor, and produces angular data. A camera senses the target position, produces a respective video data output and inputs this video data to a digital image processor. A tracker produces DC signals which are indicative of the position of the target within the field of view of the sensor. An electronic integrator produces an electronic gimbal angle, inputs the electronic gimbal angle to a motor, and the motor moves the sensor with respect to the missile.